1. Field of the Invention
The present invention relates to an information processing apparatus capable of estimating a type of ambient light.
2. Description of the Related Art
A calorimetric color reproduction-based color reproduction technique is generally used to perform color matching between different output devices (e.g., a monitor and a printer). The calorimetric color reproduction includes obtaining XYZ tristimulus values of colors output from each output device, then obtaining CIE-L*a*b* values, and finally performing color reproduction to accord L*a*b* values of colors output from respective output devices with the obtained CIE-L*a*b* values.
If the output device is a monitor or another self-luminescent device, the formula (1) shown below can be used to obtain XYZ tristimulus values of colors. On the other hand, a printed product reproduces colors by reflecting light in a viewing condition (hereinafter referred to as “ambient light”). Formula (2) can be used to obtain XYZ tristimulus values of colors in a printed product. Furthermore, formula (3) can be used to obtain CIE-L*a*b* values from XYZ tristimulus values.
As apparent from formula (2), if the ambient light for viewing a printed product is changed, the XYZ tristimulus values change correspondingly and, as a result, the CIE-L*a*b* values change. Therefore, in performing color matching between different devices (e.g., a monitor and a printer), the type of ambient light used for viewing a printed product is significant. In general, the color matching is performed by calculating XYZ tristimulus values under the reference light of sunlight D50 having a CIE (Commission Internationale de l'Éclairage)-regulated color temperature equivalent to 5000K.X=k∫visP(λ)· x(λ)·dλY=k∫visP(λ)· y(λ)·dλZ=k∫visP(λ)· z(λ)·dλk=683.0  (1)In formula (1), P(λ) represents a spectral radiance of illuminant color, and x(λ), y(λ), and z(λ) represent color matching functions.
                              X          =                      k            ⁢                                          ∫                vis                            ⁢                                                R                  ⁡                                      (                    λ                    )                                                  ·                                  P                  ⁡                                      (                    λ                    )                                                  ·                                                      x                    _                                    ⁡                                      (                    λ                    )                                                  ·                                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  Y          =                      k            ⁢                                          ∫                vis                            ⁢                                                R                  ⁡                                      (                    λ                    )                                                  ·                                  P                  ⁡                                      (                    λ                    )                                                  ·                                                      y                    _                                    ⁡                                      (                    λ                    )                                                  ·                                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  Z          =                      k            ⁢                                          ∫                vis                            ⁢                                                R                  ⁡                                      (                    λ                    )                                                  ·                                  P                  ⁡                                      (                    λ                    )                                                  ·                                                      z                    _                                    ⁡                                      (                    λ                    )                                                  ·                                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  k          =                      100                                          ∫                vis                            ⁢                                                P                  ⁡                                      (                    λ                    )                                                  ·                                                      y                    _                                    ⁡                                      (                    λ                    )                                                  ·                                                                  ⁢                                  ⅆ                  λ                                                                                        (        2        )            In formula (2), R(λ) represents a spectral reflectance of object color, P(λ) represents a spectral radiance of illuminant color, and x(λ), y(λ), and z(λ) represent color matching functions.
                                          L            *                    =                                    116              *                              f                ⁡                                  (                                      Y                                          Y                      n                                                        )                                                      -            16                          ⁢                                  ⁢                              a            *                    =                      500            *                          {                                                f                  ⁡                                      (                                          X                                              X                        n                                                              )                                                  -                                  f                  ⁡                                      (                                          Y                                              Y                        n                                                              )                                                              }                                      ⁢                                  ⁢                              b            *                    =                      200            *                          {                                                f                  ⁡                                      (                                          Y                                              Y                        n                                                              )                                                  -                                  f                  ⁡                                      (                                          Z                                              Z                        n                                                              )                                                              }                                                          (        3        )            In formula (3), Xn, Yn, and Zn represent X, Y, and Z values of a white point.
      f    ⁡          (              X                  X          n                    )        =      {                                                                                                            (                                          X                                              X                        n                                                              )                                                        1                    3                                                  ,                                                      when                    ⁢                                                                                  ⁢                                          X                                              X                        n                                                                              >                  0.008856                                                                                                                                                                                                                  7.787                    ⁢                                          (                                              X                                                  X                          n                                                                    )                                                        +                                      16                    116                                                  ,                                                      when                    ⁢                                                                                  ⁢                                          X                                              X                        n                                                                              ≤                  0.008856                                                                                                                                        ⁢                                  ⁢                  f          ⁡                      (                          Y                              Y                n                                      )                              =              {                                                                                                                                                (                                                  Y                                                      Y                            n                                                                          )                                                                    1                        3                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Y                                                      Y                            n                                                                                              >                      0.008856                                                                                                                                                                                                                                                                              7.787                        ⁢                                                  (                                                      Y                                                          Y                              n                                                                                )                                                                    +                                              16                        116                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Y                                                      Y                            n                                                                                              ≤                      0.008856                                                                                                                                                                                    ⁢                                                  ⁢                          f              ⁡                              (                                  Z                                      Z                    n                                                  )                                              =                      {                                                                                                                              (                                                  Z                                                      Z                            n                                                                          )                                                                    1                        3                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Z                                                      Z                            n                                                                                              >                      0.008856                                                                                                                                                                                                                                                                              7.787                        ⁢                                                  (                                                      Z                                                          Z                              n                                                                                )                                                                    +                                              16                        116                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Z                                                      Z                            n                                                                                              ≤                      0.008856                                                                                                                                                                                                      
However, color matching needs to be performed under various types of ambient light. Therefore, the color matching requires information about various ambient light conditions and XYZ tristimulus values obtained under each ambient light condition.
A conventional method discussed in Japanese Patent Application Laid-Open No. 2002-218266 includes measuring spectral data of ambient light with a spectral illuminometer and obtaining XYZ tristimulus values of object color under the ambient light based on spectral data of the object color and spectral data of the ambient light according to formula (2).
The method discussed in Japanese Patent Application Laid-Open No. 2002-218266 can obtain spectral data of ambient light and, accordingly, can accurately perform color matching. Furthermore, the method discussed in Japanese Patent Application Laid-Open No. 2002-218266 can hold spectral data of the object color and, therefore, can perform calculation using the spectral data.
However, if a user is required to store spectral data of object color, the volume of the spectral data is too large to process.
If sampling at intervals of 10 nm is applied to a visible light range of 380 nm to 730 nm, the spectral information of ambient light becomes a total of 36 pieces of data. Furthermore, generation of an accurate profile requires a great amount of data for several hundreds of object colors. It is therefore difficult to manage the spectral data for both the ambient light information and object colors.
On the other hand, if the color matching is performed based on only the XYZ tristimulus values of ambient light measured as ambient light information, the matching accuracy may deteriorate because the spectral data may differ even if the same XYZ tristimulus values of the ambient light are derived from formula (1). For example, fluorescent lamps are generally classified into three types: broad-band type, three-band type, and normal type. These fluorescent lamps are different in spectral data and color characteristics even if they have the same XYZ tristimulus values.
As described above, if the ambient light information used in the color matching processing is spectral data, there is a problem of too much data. If the ambient light information is XYZ tristimulus values, there is a problem of lack of accuracy.